Healthy kidneys remove waste and minerals from the blood. When kidneys fail, harmful waste builds up in the body, blood pressure may rise, and the body may retain excess fluid and not make enough red blood cells due to insufficient erythropoietin production. Hemodialysis is a common method for treating kidney failures and involves flowing blood through a filter to remove wastes. For hemodialysis, a fistula is created that connects an artery to a vein, or optionally an AV graft is created by using a tube to connect the artery to the vein. The National Kidney Foundation (NKF), Centers for Medicare and Medicaid Services (CMS), DaVita Patient Citizens (DPC) and other organizations and experts generally agree that fistulas are the best type of vascular access. After creation, a fistula can take several weeks to develop enough to be used, i.e., to reach fistula maturation. Hemodialysis can be unsuccessful or lead to complications if performed before, or too long after, fistula maturation. The evaluation of maturation by color-flow Doppler ultrasound has been proposed. See, e.g., Toregeani, et al., 2008, Evaluation of hemodialysis arteriovenous fistula maturation by color-flow Doppler ultrasound, J Vasc Bras 7(3):203-213 or Ferring, et al., 2008, Vascular ultrasound for the pre-operative evaluation prior to arteriovenous fistula formation for haemodialysis: review of the evidence, Nephrol. Dial. Transplant. 23(6):1809-1815.
Unfortunately, there are drawbacks associated with those procedures. For example, where the ultrasound catheter reveals that intravascular intervention is called for, the catheter must be swapped for a catheter that can perform the intervention (e.g., thrombectomy). Every removal and insertion of a new catheter raises risk of complications. Additionally, even though the ultrasound catheter itself provides some guidance to a treatment location, it does not provide direct navigational guidance for treatment after having been removed from the body. Further, even though medical organizations recommend fistula access, evaluating maturation is imperfect due to the fact that the ultrasound catheter being used to measure flow also partially impedes that flow. That is, the catheter itself partially occludes flow and interferes with obtaining an actual measurement of flow through an AV fistula.